Question

Coefficient of emission of an opaque body is $0.6$. Radiant heat energy is incident on it at the rate of$20watts$. Determine the quantity of radiant heat reflected by it in $2$minutes​.
A. $480J$
B. $960J$
C. $1920J$
D. $1500J$

Answer 1

Hint: In order to solve this question, we need to find the emissivity of the body given. Emissivity is the measure of degree to be able to radiate energy by a body. Use this concept to find the answer to this question.

Formula used:
$E = \varepsilon Pt$where $\varepsilon = $emissivity, $P = $Power, $t = $time taken

Complete answer:
Emissivity, $\varepsilon = 1 - 0.6$ $\therefore $because it is an opaque body.
$\varepsilon = 0.4$
We have provided $P = $$20watts$, $t = 2$minutes and $\varepsilon = 0.4$
Substituting all these value in below formula we get energy:
$ E = \varepsilon Pt \\
  E = 0.4 \times 20 \times 2 \times 60 \\
  E = 960J \\
 $(convert minutes into seconds by multiplying it by $60$)

The radiant heat energy is option(B) $960J$

Note: Points to be kept in mind while solving these questions:
Should have proper knowledge of emissivity because emissivity can be confusing sometimes.
Should learn all the important formulas as this topic contains lots of formulas which can be messy.